There is usually a valve on your gas line to the furnace you can shut off. I wouldn't worry about it though, if your furnace isn't igniting, it will stop giving it gas. @technologyconnections has a good video on how furnaces work that put my mind at ease.
Satisfying roar at the end
Is there a way to turn off the source so it's completely safe? I know nothing about furnaces and working with gas scares me
There is usually a valve on your gas line to the furnace you can shut off. I wouldn't worry about it though, if your furnace isn't igniting, it will stop giving it gas. @technologyconnections has a good video on how furnaces work that put my mind at ease.